Re: KASAN: slab-out-of-bounds Read in fbcon_get_font

From: Dmitry Vyukov
Date: Wed Dec 04 2019 - 01:34:02 EST

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On Tue, Dec 3, 2019 at 11:37 PM Daniel Vetter <daniel.vetter@xxxxxxxx> wrote:
>
> On Tue, Dec 3, 2019 at 11:25 PM syzbot
> <syzbot+4455ca3b3291de891abc@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > Hello,
> >
> > syzbot found the following crash on:
> >
> > HEAD commit: 76bb8b05 Merge tag 'kbuild-v5.5' of git://git.kernel.org/p..
> > git tree: upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=10bfe282e00000
> > kernel config: https://syzkaller.appspot.com/x/.config?x=dd226651cb0f364b
> > dashboard link: https://syzkaller.appspot.com/bug?extid=4455ca3b3291de891abc
> > compiler: gcc (GCC) 9.0.0 20181231 (experimental)
> > syz repro: https://syzkaller.appspot.com/x/repro.syz?x=11181edae00000
> > C reproducer: https://syzkaller.appspot.com/x/repro.c?x=105cbb7ae00000
> >
> > IMPORTANT: if you fix the bug, please add the following tag to the commit:
> > Reported-by: syzbot+4455ca3b3291de891abc@xxxxxxxxxxxxxxxxxxxxxxxxx
> >
> > ==================================================================
> > BUG: KASAN: slab-out-of-bounds in memcpy include/linux/string.h:380 [inline]
> > BUG: KASAN: slab-out-of-bounds in fbcon_get_font+0x2b2/0x5e0
> > drivers/video/fbdev/core/fbcon.c:2465
> > Read of size 16 at addr ffff888094b0aa10 by task syz-executor414/9999
>
> So fbcon allocates some memory, security/tomoyo goes around and frees
> it, fbcon goes boom because the memory is gone. I'm kinda leaning
> towards "not an fbcon bug". Adding relevant security folks and mailing
> lists.
>
> But from a very quick look in tomoyo it loosk more like "machine on
> fire, random corruption all over". No idea what's going on here.

Hi Daniel,

This is an out-of-bounds access, not use-after-free.
I don't know why we print the free stack at all (maybe +Andrey knows),
but that's what KASAN did from day one. I filed
https://bugzilla.kernel.org/show_bug.cgi?id=198425 which I think is a
good idea, I will add your confusion as a data point :)
Re this bug, free stack is irrelevant, I guess it's when the heap
block was freed before it was reallocated by console. So it's plain
out-of-bounds in fbcon_get_font, which looks sane and consistent to me
and reproducible on top.

> > CPU: 0 PID: 9999 Comm: syz-executor414 Not tainted 5.4.0-syzkaller #0
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> > Google 01/01/2011
> > Call Trace:
> > __dump_stack lib/dump_stack.c:77 [inline]
> > dump_stack+0x197/0x210 lib/dump_stack.c:118
> > print_address_description.constprop.0.cold+0xd4/0x30b mm/kasan/report.c:374
> > __kasan_report.cold+0x1b/0x41 mm/kasan/report.c:506
> > kasan_report+0x12/0x20 mm/kasan/common.c:638
> > check_memory_region_inline mm/kasan/generic.c:185 [inline]
> > check_memory_region+0x134/0x1a0 mm/kasan/generic.c:192
> > memcpy+0x24/0x50 mm/kasan/common.c:124
> > memcpy include/linux/string.h:380 [inline]
> > fbcon_get_font+0x2b2/0x5e0 drivers/video/fbdev/core/fbcon.c:2465
> > con_font_get drivers/tty/vt/vt.c:4446 [inline]
> > con_font_op+0x20b/0x1250 drivers/tty/vt/vt.c:4605
> > vt_ioctl+0x181a/0x26d0 drivers/tty/vt/vt_ioctl.c:965
> > tty_ioctl+0xa37/0x14f0 drivers/tty/tty_io.c:2658
> > vfs_ioctl fs/ioctl.c:47 [inline]
> > file_ioctl fs/ioctl.c:545 [inline]
> > do_vfs_ioctl+0x977/0x14e0 fs/ioctl.c:732
> > ksys_ioctl+0xab/0xd0 fs/ioctl.c:749
> > __do_sys_ioctl fs/ioctl.c:756 [inline]
> > __se_sys_ioctl fs/ioctl.c:754 [inline]
> > __x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:754
> > do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
> > entry_SYSCALL_64_after_hwframe+0x49/0xbe
> > RIP: 0033:0x4444d9
> > Code: 18 89 d0 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 00 48 89 f8 48 89 f7
> > 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff
> > ff 0f 83 7b d8 fb ff c3 66 2e 0f 1f 84 00 00 00 00
> > RSP: 002b:00007fff6f4393b8 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
> > RAX: ffffffffffffffda RBX: 00007fff6f4393c0 RCX: 00000000004444d9
> > RDX: 0000000020000440 RSI: 0000000000004b72 RDI: 0000000000000005
> > RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000400da0
> > R10: 00007fff6f438f00 R11: 0000000000000246 R12: 00000000004021e0
> > R13: 0000000000402270 R14: 0000000000000000 R15: 0000000000000000
> >
> > Allocated by task 9999:
> > save_stack+0x23/0x90 mm/kasan/common.c:71
> > set_track mm/kasan/common.c:79 [inline]
> > __kasan_kmalloc mm/kasan/common.c:512 [inline]
> > __kasan_kmalloc.constprop.0+0xcf/0xe0 mm/kasan/common.c:485
> > kasan_kmalloc+0x9/0x10 mm/kasan/common.c:526
> > __do_kmalloc mm/slab.c:3656 [inline]
> > __kmalloc+0x163/0x770 mm/slab.c:3665
> > kmalloc include/linux/slab.h:561 [inline]
> > fbcon_set_font+0x32d/0x860 drivers/video/fbdev/core/fbcon.c:2663
> > con_font_set drivers/tty/vt/vt.c:4538 [inline]
> > con_font_op+0xe18/0x1250 drivers/tty/vt/vt.c:4603
> > vt_ioctl+0xd2e/0x26d0 drivers/tty/vt/vt_ioctl.c:913
> > tty_ioctl+0xa37/0x14f0 drivers/tty/tty_io.c:2658
> > vfs_ioctl fs/ioctl.c:47 [inline]
> > file_ioctl fs/ioctl.c:545 [inline]
> > do_vfs_ioctl+0x977/0x14e0 fs/ioctl.c:732
> > ksys_ioctl+0xab/0xd0 fs/ioctl.c:749
> > __do_sys_ioctl fs/ioctl.c:756 [inline]
> > __se_sys_ioctl fs/ioctl.c:754 [inline]
> > __x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:754
> > do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
> > entry_SYSCALL_64_after_hwframe+0x49/0xbe
> >
> > Freed by task 9771:
> > save_stack+0x23/0x90 mm/kasan/common.c:71
> > set_track mm/kasan/common.c:79 [inline]
> > kasan_set_free_info mm/kasan/common.c:334 [inline]
> > __kasan_slab_free+0x102/0x150 mm/kasan/common.c:473
> > kasan_slab_free+0xe/0x10 mm/kasan/common.c:482
> > __cache_free mm/slab.c:3426 [inline]
> > kfree+0x10a/0x2c0 mm/slab.c:3757
> > tomoyo_init_log+0x15c1/0x2070 security/tomoyo/audit.c:294
> > tomoyo_supervisor+0x33f/0xef0 security/tomoyo/common.c:2095
> > tomoyo_audit_env_log security/tomoyo/environ.c:36 [inline]
> > tomoyo_env_perm+0x18e/0x210 security/tomoyo/environ.c:63
> > tomoyo_environ security/tomoyo/domain.c:670 [inline]
> > tomoyo_find_next_domain+0x1354/0x1f6c security/tomoyo/domain.c:876
> > tomoyo_bprm_check_security security/tomoyo/tomoyo.c:107 [inline]
> > tomoyo_bprm_check_security+0x124/0x1a0 security/tomoyo/tomoyo.c:97
> > security_bprm_check+0x63/0xb0 security/security.c:784
> > search_binary_handler+0x71/0x570 fs/exec.c:1645
> > exec_binprm fs/exec.c:1701 [inline]
> > __do_execve_file.isra.0+0x1329/0x22b0 fs/exec.c:1821
> > do_execveat_common fs/exec.c:1867 [inline]
> > do_execve fs/exec.c:1884 [inline]
> > __do_sys_execve fs/exec.c:1960 [inline]
> > __se_sys_execve fs/exec.c:1955 [inline]
> > __x64_sys_execve+0x8f/0xc0 fs/exec.c:1955
> > do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
> > entry_SYSCALL_64_after_hwframe+0x49/0xbe
> >
> > The buggy address belongs to the object at ffff888094b0a000
> > which belongs to the cache kmalloc-4k of size 4096
> > The buggy address is located 2576 bytes inside of
> > 4096-byte region [ffff888094b0a000, ffff888094b0b000)
> > The buggy address belongs to the page:
> > page:ffffea000252c280 refcount:1 mapcount:0 mapping:ffff8880aa402000
> > index:0x0 compound_mapcount: 0
> > raw: 00fffe0000010200 ffffea0002a3ae08 ffffea0002a6aa88 ffff8880aa402000
> > raw: 0000000000000000 ffff888094b0a000 0000000100000001 0000000000000000
> > page dumped because: kasan: bad access detected
> >
> > Memory state around the buggy address:
> > ffff888094b0a900: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> > ffff888094b0a980: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> > > ffff888094b0aa00: 00 00 fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> > ^
> > ffff888094b0aa80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> > ffff888094b0ab00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> > ==================================================================
> >
> >
> > ---
> > This bug is generated by a bot. It may contain errors.
> > See https://goo.gl/tpsmEJ for more information about syzbot.
> > syzbot engineers can be reached at syzkaller@xxxxxxxxxxxxxxxxx
> >
> > syzbot will keep track of this bug report. See:
> > https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
> > syzbot can test patches for this bug, for details see:
> > https://goo.gl/tpsmEJ#testing-patches

Next message: syzbot: "general protection fault in gcmaes_crypt_by_sg (2)"
Previous message: Dan Carpenter: "Re: [PATCH -next] power: supply: ab8500: Drop pointless static qualifier in ab8500_btemp_batctrl_volt_to_res()"
In reply to: Daniel Vetter: "Re: KASAN: slab-out-of-bounds Read in fbcon_get_font"
Next in thread: Daniel Vetter: "Re: KASAN: slab-out-of-bounds Read in fbcon_get_font"
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